The present invention relates to an arrangement for determining the position of a laser beam in a scattering medium.
Such arrangements are required in a number of technical applications, for example, in guide beam processes for the guidance of flying bodies or for landing approach control. In particular, such methods require precise determination of the position of the laser beam even in scattering media, for example, smog, fog, smoke or water and the angular precision of this determination should be of the order of magnitude of the original beam divergence of the laser beam, which means, in the range of a few milliradians. For example, in a guide beam process for guiding a flying body, the laser beam scans the surroundings of the flying body in a particular pattern. On the basis of the time, at which the laser beam sweeps over a sensor on the back of the flying body, the position of the flying body relative to a target can be determined in the flying body. This forms the basis for obtaining data for guiding the flying body. In addition, it is possible to modulate the laser beam within a given range in order to transmit in this way commands to the flying body, for example, controlling signals. When a continuous wave laser is used, this as well as other methods can essentially only be used up to a standard meteorological range of sight of 1 to 2 km. If the optical range is less, the laser beam is scattered in the surrounding medium to such an extent that definite position finding is no longer possible. This fact is briefly explained:
If a laser beam is lased through a scattering medium, for example, smog or fog, a fraction of the laser irradiation is scattered, weakening the original laser beam. The radiation registered by a receiver I is composed additively of an unscattered coherent fraction I.sub.c and the incoherent scattered fraction I.sub.i, i.e. the relationship EQU p I=I.sub.c +I.sub.i
applies.
If the starting intensity of the laser beam is called I.sub.0, then the coherent content is EQU I.sub.c =I.sub.0 exp (-.tau.)
where .tau. is the optical depth or thickness of the medium and given by EQU .tau.=L.sigma..sub.e
where L is the geometric distance between the laser transmitter and the receiver and .theta.e the extinction coefficient of the medium. The relationship EQU .sigma..sub.e =.sigma..sub.s +.sigma..sub.a
applies, where .sigma..sub.s is the scattering coefficient and .sigma..sub.a the absorption coefficient of the medium.
The scattered content I.sub.i is a function of the scattering coefficient .sigma..sub.s and of the scattering characteristic of the particles of the scattering medium. If the wavelength of the laser is smaller or comparable to the diameter of the scattering particles in the medium, then the scattered radiation is directed forward and spreads in an angular range of a few degrees around the axis of the laser beam.
If the wavelength of the laser beam is greater than the diameter of the scattering particles, the scattered radiation spreads over a wider angular range.
Compared to the coherent radiation, however, the beam divergence of which is, in general, of the order of magnitude within the milliradian range, a background always originates, in which the original and coherent laser beam disappears with increasing optical depth and thickness .tau.. This is evident in FIG. 1, in which the course of the beam profile of a laser beam with aerosol scattering as a function of distance .rho. in cm from the beam axis for different scattering coefficients .sigma..sub.s and a distance of 3 km between laser transmitter and laser receiver is plotted. The beam profiles are standardized to the beam axis and indicated logarithmically as radiation intensity. It is obvious that even with a relatively small scattering coefficient the beam profile is spread to such an extent that a somewhat exact determination of the position of the laser beam is no longer possible.
Since the scattered radiation has the same wavelength as the coherent radiation, they can not be differentiated from each other with optical filters. With a receiver based on conventional designs, therefore, at optical depths .tau..gtorsim.10 corresponding to a standard meteorological range of approximately 500 m only very imprecise determinations of the position of the laser beam depending on the angular distribution of the scattered radiation are still possible. For the technical applications mentioned above such inexact position finding of the laser beam is no longer tolerable.